Lithium (Li) ion secondary batteries are, for example, configured to have a positive electrode, a negative electrode formed of a negative electrode active material layer on the surface of a negative electrode current collector, and a non-aqueous electrolyte, and are used in mobile phones, notebook type personal computers, and the like.
For example, the negative electrode of the lithium ion secondary battery is formed by coating, drying, and then pressing carbon particles as the negative electrode active material layer on the surface of a negative electrode current collector of which both surfaces are formed of a flat and smooth copper foil.
As the negative electrode current collector formed of copper foil described above, a negative electrode current collector manufactured by electrolysis is used, that is, a negative electrode current collector where an “unprocessed copper foil” is subjected to rust proofing.
As the negative electrode active material for lithium ion secondary batteries, development is underway of next generation negative electrode active materials having a charge and discharge capacity which greatly exceeds the theoretical capacity of carbon material.
For example, materials including metal capable of alloying with lithium such as silicon (Si), tin (Sn), or the like are expected.
However, in a case of using these active materials, since the changes in volume in accordance with the storage and release of lithium during charging and discharging are large, there are cases where it is difficult to favorably maintain an adhesion state between the current collector and the active material. As a result, the current collector is damaged and the cycle properties deteriorate. As countermeasures, setting the tensile strength of the current collector to a predetermined value or greater, or setting the elongation to a predetermined value or greater have been reported.
For example, a copper foil provided with a polyimide binder for improving the adhesion of the active material and the current collector and which suppresses the disadvantages described above is being developed.
However, the expansion and contraction of the active material is a micro-level phenomenon since the units thereof are the active material particles and the expansion and contraction of the active material does not have a simple correlation with the macro-level characteristics such as the tensile characteristics of the current collector. For this reason, it is difficult to further improve the cycle properties.
Patent Documents 1 to 12 describe an electrodeposited copper foil used in a negative electrode current collector of a lithium ion secondary battery, or the like.